1. Field of the Invention
This invention relates to porous polyethylene fibers having very light weight and a soft feeling.
2. Prior Art
In recent years, the diversty of fibers for use in clothing has increased greatly. As a part of this diversity, there is a growing demand for fibers having lighter weight and a softer feeling.
Fibers in ordinary form have a limit in light-weight properties, depending on the material. If crimping is used, the resulting fibers inevitably have a feeling characteristic of crimped fibers. The same is the case with soft feeling. Thus, fibers made on different principles are being required for purposes of diversity.
In order to meet this demand, the present inventors attempted to develop a new material comprising porous fibers.
A variety of porous fibers have been proposed in the prior art. They include, for example, those prepared by melt-spinning a blend of a thermoplastic polymer and a blowing agent, and decomposing the blowing agent during spinning to make the spun fibers porous; those prepared by melt-spinning a blend of a thermoplastic polymer and another component such as inorganic fine particles or an incompatible polymer, and then stretching the spun fibers to form empty spaces at the interface between the thermoplastic polymer and the other component; those prepared by spinning a blend of a thermoplastic polymer and an extractable substance, and then extracting the extractable substance with a suitable solvent to produce pores; and those prepared by forming polyester filaments having a specific structure and treating them with an amine and an alkali to produce a porous structure (as in Japanese Patent Laid-Open No. 179369/'86).
However, the process using a blowing agent fails to yield porous fibers of consistent quality, probably because the spinning step has poor stability. If an attempt is made to enhance the porosity, fiber breakage occurs frequently and a marked reduction in strength results. Thus, it is impossible to obtain fibers having both high porosity and high strength. The process using inorganic fine particles or an incompatible polymer to prepare porous fibers has the disadvantage that it is difficult to blend such an additive uniformly with the thermoplastic polymer. If a large amount of additive is added in order to enhance the porosity, the additive prevents full orientation of the sea component constituting the fibers proper, making it impossible to obtain porous fibers having high strength. Thus, this process also fails to achieve the desired combination of high porosity and high strength. The extraction process is also disadvantageous in that it involves complicated steps which raise the cost of the fibers and, as in the above-described processes, it is impossible to obtain porous fibers having high porosity and high strength. The process described in Japanese Patent Laid-Open No. 179369/'86 involves complicated steps and, moreover, cannot be applied to materials other than polyesters. Furthermore, judging from the examples described therein, even fibers having a porosity of as low as 35-45% exhibit a tensile strength of 2.9 g/d or less. Thus, the desired combination of high porosity and high strength again cannot be achieved.
A process for preparing porous fibers by melt spinning and stretching is disclosed in U.S. Pat. No. 3,549,743. It is described therein that porous polypropylene fibers can be prepared by this process, but the fibers thus obtained have an apparent density of 50 to 85% and hence a porosity of 15 to 50%. Thus, no fibers having a porosity greater than 50% are disclosed therein.
A similar process for preparing porous polyethylene hollow fibers by melt spinning and stretching is disclosed in U.S. Pat. No. 4,401,567. However, those fibers have larger diameters (i.e., not less than 50 .mu.m in inner diameter and not less than 70 .mu.m in outer diameter) than ordinary fibers. Although it is known that hollow fibers having such large diameters can be obtained, it is not easy to prepare ordinary fibers having smaller diameters. More specifically, in preparing porous fibers according to the process disclosed in U.S. Pat. No. 4,401,567, it is necessary to obtain unstretched fibers having a high degree of crystal orientation. To this end, a high-density polyethylene having a relatively low melt index is subjected to high-draft spinning at a temperature lower than the usual spinning temperature. Accordingly, in order to obtain ordinary fibers having a smaller diameter, higher-draft spinning conditions must be established by either sharply increasing the spinning speed or sharply decreasing the extrusion rate.
Under these conditions, however, fiber breakage tends to occur just under the spinneret owing to the marked increase in tension, resulting in reduced spinning stability. Moreover, since the elongation of the unstretched fibers is markedly reduced, high stretching ratios cannot be established in the stretching step. Thus, it is difficult to achieve a high porosity of 50% or greater.
On the other hand, as disclosed in U.S. Pat. No. 3,549,743, polypropylene can be relatively stably spun to obtain unstretched fibers which have a small diameter and can be made porous. However, the porous polypropylene fibers so prepared have smaller micropores than porous polyethylene fibers. If the stretching ratio is increased, the rearrangement of molecular chains proceeds to cause the collapse of micropores and hence a reduction in porosity. Thus, it is again difficult to obtain porous fibers having a porosity of 50% or greater.
Thus, although polyolefins are materials suitable for the manufacture of healthful clothing, polyethylene is not used as a clothing material because of its characteristic waxy feeling. In view of those circumstances, the present inventors conducted an intensive study to greatly diminish the waxy feeling of polyethylene that is an inherently lightweight material, and thereby develop a novel material being very light weight an having high intensity. The present invention was completed as a result of this study.